The transmission seems to be from something prior to say 1955. I think I can see the prongs that are on the output shaft which mate with what is called a Hardy disk...it softens the transfer of motion from the transmission to the drive shaft. Plus I don't recall seeing the clutch actuation arm sticking up high like that...normally it goes down low. It could even be a non-BMW transmission from that era.

Well, the serial number would tell you what the original displacement was.
Who knows what the internals look like.

$25? $50?

What is it worth, to you? Do you need an engine?

And unless it is an R45 that found it's way over from Germany/UK, the smallest it could be is 650cc.
Who is this seller that doesn't know his airhead parts?
Must be cleaning out a deceased relative's shed.

To me about the only parts on there you'd ever be able to sell or use would be the valve covers, starter, alternator and diode board. Not knowing the displacement rules out reaping any gain from the cylinders and heads. They might be icing on the cake, but you can't count on that.

Also, working on your own vehicle is almost a lost art. The number of people seeing that ad who own their own wrenches can probably be counted on 5 fingers. Offer him $80 cash in person.

After 20 years as a professional bike mechanic and 30 years as an engineer I know just enough to be dangerous !